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From "Radim Kolar (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-4827) Increase hash quality of HashPartitioner
Date Wed, 28 Nov 2012 17:52:58 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-4827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13505707#comment-13505707
] 

Radim Kolar commented on MAPREDUCE-4827:
----------------------------------------

its knutt formula commonly used in hashtables for improve hashing. java hashtable is using
it too
                
> Increase hash quality of HashPartitioner
> ----------------------------------------
>
>                 Key: MAPREDUCE-4827
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-4827
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>            Reporter: Radim Kolar
>         Attachments: betterhash1.txt
>
>
> hash partitioner is using object.hashCode() for splitting keys into partitions. This
results in bad distributions because hashCode() quality is poor. 
> These hashCode() functions are sometimes written by hand (very poor quality) and sometimes
generated from by commons lang code (poor quality). Applying some transformation on top of
hashCode() provides better distribution.

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